1. Field of the Invention
The present invention relates to a print position adjustment method and a printing apparatus and a printing system using the print position adjustment method, and is particularly suited for adjusting the positions of ink dots in a printing apparatus of an ink jet system. In addition to general printing apparatus, the present invention can also be applied to copying machines, facsimiles with a communication system, word processors with a printer, and industrial printing apparatus combined with a variety of processing devices.
2. Description of the Related Art
An image printing apparatus of a so-called serial scan type, which executes the print operation while scanning a print head, or a printing unit, over a print medium, has found a variety of image forming applications. The ink jet printing apparatus in particular has in recent years achieved high resolution and color printing, making a significant image quality improvement, which has resulted in a rapid spread of its use. Such an apparatus employs a so-called multi-nozzle head that has an array of densely arranged nozzles for ejecting ink droplets. Images with still higher resolution have now been made possible by increasing the nozzle density and reducing the amount of ink per dot. Further, to realize an image quality approaching that of a silver salt picture, various technologies have been developed, including the use of pale or light color ink with reduced concentration in addition to four basic color inks (cyan, magenta, yellow and black). A print speed reduction problem, which is feared to arise as the picture quality advances, is dealt with by increasing the number of print elements, improving the drive frequency and employing a bi-directional printing technique, thus realizing a satisfactory throughput.
FIG. 27 schematically shows a general construction of a printer that uses the multi-nozzle for printing. In the figure, reference number 1901 represents head cartridges corresponding to four inks, black (K), cyan (C)), magenta (M) and yellow (Y). Each head cartridge 1901 consists of an ink tank 1902T filled with a corresponding color ink and a head unit 1902H having an array of many nozzles for ejecting the ink supplied from the ink tank onto a print medium 1907.
Designated 1903 is a paper feed roller which, in cooperation with an auxiliary roller 1904, clamps a print medium (print paper) 1907 and rotates in the direction of arrow in the figure to feed the print paper 1907 in the Y direction as required. Denoted 1905 is a pair of paper supply rollers that clamps the print paper 1907 and carries it toward the print position. The paper supply rollers 1905 also keep the print paper 1907 flat and tight between the supply rollers and the feed rollers 1903, 1904.
Designated 1906 is a carriage that supports the four head cartridges 1901 and moves them in a main scan direction during the print operation. When the printing is not performed or during an ink ejection performance recovery operation for the head unit 1902H, the carriage 1906 is set at a home position h indicated by a dotted line.
The carriage 1906, which was set at the home position h before the print operation, starts moving in the X direction upon reception of a print start command and at the same time the head unit 1902H ejects ink from a plurality of nozzles (n nozzles) formed therein according to print data to perform printing over a band of a width corresponding to the length of the nozzle array. When the printing is done up to the X-direction end of the print paper 1907, the carriage 1906 returns to the home position h in the case of one-way printing and resumes printing in the X direction. In the case of bi-directional printing, the carriage 1906 also performs printing while it is moving in a −X direction toward the home position h. In either case, after one print operation (one scan) in one direction has been finished before the next print operation is started, the paper feed roller 1903 is rotated a predetermined amount in the direction of arrow in the figure to feed the print paper 1907 in the Y direction a predetermined distance (corresponding to the length of the nozzle array). By repeating the one-scan print operation and the print paper feeding by a predetermined distance, data for one sheet of paper is printed.
In the above serial type ink jet printer, various provisions have been made as to the construction of the head unit or the printing method in order to realize an image printing with higher resolution.
For example, the manufacture of the multi-nozzle head inevitably places a limit on the density of the nozzles in a single nozzle array.
FIG. 28A shows an example head that realizes a higher recording density. This head has two columns of nozzles extending in the Y direction and spaced a distance px (corresponding to a predetermined number of pixels) apart in the X direction. The two nozzle columns, each consisting of many nozzles arranged at a predetermined pitch py in the Y direction, are shifted from each other by a distance py/2 in the Y direction. This arrangement of the nozzles realizes a resolution two times higher than that achieved by a single nozzle column. When this head is applied to the apparatus shown in FIG. 27, the heads having the construction shown in FIG. 28A for one color can be arranged in parallel in the X direction for six colors. In this arrangement, simply adjusting the ejection timings of the two nozzle columns can achieve a color printing with two times the resolution of the single nozzle column.
In other technologies, such as U.S. Pat. No. 4,920,355 and Japanese Patent Application Laid-Open No. 7-242025 (1995), a high resolution printing is realized by setting the paper feed distance for each print scan to a predetermined number of pixels less than the length of the column of nozzles while leaving the multi-nozzle arrangement at a low resolution. Such a printing method is hereinafter called an interlace printing method.
The interlace printing method will be briefly explained by referring to FIG. 29. Here let us take up an example case where an image with resolution of 1200 DPI (dots/inch) is printed by using a head H with nozzles arranged at a pitch of 300 DPI. For the sake of simplicity, it is assumed that the head has nine nozzles and that the distance of the paper feed carried out after each print scan is nine pixels at 1200-DPI resolution. The rasters printed in the forward pass are shown as solid lines and the rasters printed in the backward pass are shown as dashed lines. These two kinds of lines are formed alternately.
While in this example the paper is fed a fixed distance of 9 pixels at 1200-DPI resolution, other arrangements may be made in the interlace printing. The interlace printing method does not need to have a constant paper feed distance at all times as long as a picture is printed with a plurality of print scans arranged at a pitch finer than the arrangement pitch can be printed with a higher resolution than the nozzle arrangement resolution.
When a head as shown in FIG. 28A is used, because even-numbered rasters and odd-numbered rasters that are alternated in the Y direction (sub-scan direction) are printed by different columns of nozzles, the landing positions of ink droplets from the two columns of nozzles may deviate subtly from the correct positions, degrading the image quality. One of possible causes for this problem may be explained as follows. When a head face on which nozzles are formed is deformed due to swelling with ink or temperature rise, causing a part of the head face between the nozzle column associated with the odd-numbered rasters and the nozzle column associated with the even-numbered rasters to bulge, as shown in FIG. 28B, the ink droplets from the respective nozzle columns will be projected in two different directions slightly away from each other. The ink landing position deviation between the rasters due to this phenomenon, even if small in magnitude, will have bad effects on the image quality and pose a critical problem in realizing a high resolution photographic image quality, one of the objects of the present invention.
Many proposals have been put forward as to the method of correcting ink landing position deviations among different colors and, in the bi-directional printing, the method of correcting deviations in ink landing position of the same color between the forward scan and the backward scan. However, as for the correction of the ink landing position deviations between the rasters of the same color produced by the head shown in FIG. 28A, an effective adjustment method has yet to be proposed although the allowable range for the deviation is narrow and the effects of such deviations on the image formation are large. Further, the deviation in ejection direction between the even-numbered nozzle column and the odd-numbered nozzle column is caused by the ink composition, ink ejection history such as ejection frequency, and printing environment, as well as the characteristic variations of individual heads. Therefore, even if the ink ejection timing for a head is determined which does not cause ink landing position deviations under a particular condition, that ejection timing cannot be applied to all circumstances. That is, not only should the ink ejection timing be adjusted before shipping according to the characteristic variations of individual heads, it is also strongly called for that the adjustment be able to be made as required demands being met, it is difficult to form a high quality image at all times.
Further, in the interlace printing method, because the same image area is completed by repeating the print scan and the paper feed a plurality of times, the printing time will increase. To cope with this problem, a bi-directional printing has been proposed and disclosed. In this case, the odd-numbered rasters are often printed by the forward scans and the even-numbered rasters by backward scans, as shown in FIG. 29. If the ink landing positions deviate from one raster to another, the similar problem to that when the head of FIG. 28A is used will occur.
There are many proposals already put forth as to the method of correcting ink landing position deviations between forward scan and backward scan. The proposed methods mostly take note of a vertical line pattern where the same image area is completed by a single scan (one pass printing), and do not address the problem of correcting subtle deviations among the rasters when performing the interlace printing.